The present invention relates to steered laser beam systems that perform work on a workpiece, and more particularly to a method for operating a steered laser beam system employing selective laser power control based on a velocity of the laser beam focal point.
Laser systems are presently being used as supplements or alternatives to other machining processes. Typically, such laser systems use high-power laser beams for cutting, welding and surface treatment of materials. Laser beam cutting is presently considered a standard industrial process.
In many applications, the table on which the workpiece(s) is disposed is moved in two dimensions with respect to a laser beam that is stationary. This technology is often referred to as an "x-y table" laser system. However, such systems are typically only able to achieve relatively slow movement, and therefore are inefficient for many applications.
Steered or directed laser beam systems have been developed for applications in which workpieces need to be processed at a high speed to be economical. Such steered or directed laser beams are well known and use linear motors or operate using a "galvo system". A galvo system employs two or more mirrors for reflecting the laser beam in a controlled path to selectively position the focal point of the laser beam on the workpiece. The reflective angles of the mirrors are adjusted, typically under computer control, to alter the position of the focal point of the laser beam in two (or more) dimensions.
FIG. 1 is a diagram simplistically illustrating a typical prior art galvo steered laser beam system 10. Galvo system 10 includes laser source 12 emitting laser beam 14. Galvo mirrors 16 and 18 are provided and arranged to control the "x" (horizontal) position and the "y" (vertical) position, respectively, of the focal point of laser beam 14 on workpiece 20. This position control is achieved by adjusting the reflectance angles of mirrors 16 and 18. For an application involving complex cutting pattern, mirrors 16 and 18 are typically under computer control, and velocities of the focal point of laser beam 14 may exceed 100 in/sec in some applications.
One problem associated with using a steered laser beam system in a cutting process is obtaining a uniform depth of cut or a uniform score when such systems are operated at high speeds, where the focal point is moving at times with velocities above about 100 in/sec. The basis of the problem is the fact that at high speeds, a uniform focal point velocity cannot be maintained for cutting patterns having shaped features such as curves and corners. The focal point velocity must decrease in order to traverse curves and comers, and then is able to return to full velocity on straight portions of the cutting pattern. The difficulty of the non-uniform velocity profile is that with uniform power applied to the laser beam, a deeper cut will be made where the laser beam focal point is traveling at lower velocities (corners and curves) than will be made where the laser beam focal point is traveling at higher velocities (straight portions). In addition, at the beginning of a cut, the galvo mirrors are not able to instantaneously reach a high enough velocity to turn the laser on, and when the laser finally does turn on, the laser power tends to spike up to an undesirably high power level, resulting in too deep and wide of a cut at the starting location.
An example of an application where the depth of cut is important is a multi-layer film such as a decal. The decal must be cut through the adhesive material of the decal itself without cutting through the backing of the decal. FIG. 2 illustrates an example of an application performed by a prior art steered laser beam system, involving a simple rounded square cut of a multi-layer film such as a decal. Where uniform power was applied to the laser beam throughout cutting pattern 30, the backing of the decal was cut through in corner regions 32. This occurred because the velocity of the laser beam decreased in corner regions 32, causing the laser beam to be focused on a particular point in corner regions 32 lightly longer than for the straight portions of cutting pattern 30. As a result, the cut in corner regions 32 was deeper and wider than in the straight portions of cutting pattern 30, and the backing of the decal was undesirably cut through in corner regions 32. With existing technology, the only solution to this problem is to manually pre-program the desired laser power levels for each portion of the cutting pattern. This solution is extremely inefficient and time-consuming, particularly for cutting patterns having more complex geometries than the simple cutting pattern pictured in FIG. 2, and also does not address problematic power spikes that may occur when initially turning on the laser.
In laser systems that move on an x-y table, steps have been taken to anticipate approaching corners or starting score lines, to combat the non-uniform cut depth problem. Since x-y tables move at slow rates, one solution has been to pulse the laser beam, that is to turn it on and off intermittently. Intermittent pulsing of the laser works well in an x-y table application because the x-y table includes digital position feedback that can be used directly control the laser pulsing duty cycle, and also because of the relatively low velocities of movement involved. However, pulsing a laser beam in a high-speed steered laser beam system is not a viable solution due to optical and mechanical delays associated with delivering laser energy to the focal point, which become prohibitive problems at high focal point velocities, and because there is no position feedback signal to control a duty cycle of laser pulsing in a galvo laser system.
U.S. Pat. Nos. 5,340,962 and 5,428,280 by Schmidt et al. describe a laser machining system that controls the distance of the focal point of the laser beam from the machining nozzle tip to provide a constant distance from the tip to the workpiece to provide satisfactory cuts or welds. However, such a system does not address the problem of steering a laser beam at different cutting or welding speeds while providing a uniform depth cut or weld.
U.S. Pat. No. 4,160,894 by Stemmler et al. describes a steered laser beam system for cutting two-dimensional patterns on a moving web of material through the use of a lens that is supported on a rotatable movable lens carrier on which the lens is fixed eccentrically to the axis of rotation. However, the quality of the cut being produced by the beam is not addressed since the application is a through cut.
Therefore, there is a need in the art for a system that is able to achieve a uniform depth of cut in a high-speed steered laser system, despite nonuniform velocities of the focal point of the laser beam as it traverses a cutting pattern.